1. Field of the Invention
Embodiments of the present invention relate to a method and apparatus for controlling vapor pressure in a sorption analyzer. More particularly, embodiments of the present invention relate to a method and apparatus for controlling and maintaining the atmosphere surrounding samples undergoing sorption analysis to a very low partial vapor pressure.
2. Background Information
A sorption analyzer is an instrument that can create and maintain a desired atmosphere within a chamber and measure a property of a sample placed in the chamber. For example, the partial vapor pressure within a chamber can be created and maintained for the purpose of studying a sample's tendency to absorb or desorb a vapor. Vapors can include but are not limited to water vapor or vapor from an organic solvent.
The weight of a sample is one property that can be measured by a sorption analyzer. The amount of absorption or desorption of a vapor by a sample can be determined by weighing the sample at various partial vapor pressures, for example. One type of sorption analyzer utilizes a microbalance in the chamber of the sorption analyzer for weighing the sample. A sorption analyzer that determines the absorption or desorption of a vapor by a sample by measuring the weight of the sample performs gravimetric sorption analysis.
A sorption analyzer can also create and maintain the temperature within a chamber. A thermogravimetrical analyzer (TGA) is an instrument generally used to measure the decomposition of a sample as a function of temperature. A TGA heats a sample and measures the weight gain or loss during the process. TGAs are described, for example, in U.S. Pat. No. 5,165,792, which is incorporated by reference herein. TGAs can be adapted to provide gravimetrical sorption analysis or thermogravimetrical sorption analysis. A humidity-controlled chamber used in conjunction with a TGA to provide sorption analysis is described, for example, in U.S. patent application Ser. No. 10/910,575 filed Aug. 4, 2004, which is incorporated by reference herein.
Some hygroscopic materials require extremely low partial vapor pressures to completely dry prior to sorption analysis. For example, microcrystalline cellulose (MCC) BCR RM 302 requires an extremely low partial vapor pressure of water to completely dry prior to performing sorption analysis according to the European cost 90 method. In view of the foregoing, it can be appreciated that a substantial need exists for methods and apparatus that can control and maintain the atmosphere surrounding samples undergoing sorption analysis to a very low partial vapor pressure.